In a transmission system to which an optical signal is applied, it is preferable that information can be transmitted and received in addition to a main signal between a transmitter and a receiver in a simple configuration. Japanese Laid-open Patent Publication No. 2002-271440 discloses a technique in which a transmitter transmits a low frequency ON/OFF signal, in addition to a main signal, to a receiver. Japanese Examined Utility Model Application Publication No. 8-4760 discloses a technique in which transmission/non-transmission to an optical fiber is performed by specifying drive/non-drive of a modulation unit according to a signal.